1. Field of the Invention
The present invention relates to a displacement element suitable for use in a tunnel current detecting device, a scanning tunneling microscope or the like, and to a cantilever probe employing this element. The present invention particularly relates to a large-capacity and high-density information processing apparatus for recording, reproducing and erasing information by using the cantilever probe through the technology of a scanning tunneling microscope.
2. Description Of the Related Art
In recent years, electromechanical elements (micromechanics), such as semiconductor pressure sensors, semiconductor acceleration sensors, microactuators or the like using semiconductors as mechanical structures have attracted attention as applied to a background of semiconductor process technology.
Such elements are desirable because they are compact and high-precision mechanical parts are readily available. Since a semiconductor wafer is used, elements and electrical circuits can be formed integrally on a Si wafer. An increase in productivity through batch operations of a semiconductor process can be expected by manufacturing such elements based on such semiconductor processes. An example of a very small displacement element is a cantilevered one using an electrostatic force or a piezoelectric thin film. Since very small movements of this element can be finely controlled, this element is applied to a scanning tunneling microscope (hereinafter referred to as a STM) capable of directly observing atomic and molecular levels.
A cantilever using an electrostatic force has been proposed in, for example, U.S. Pat. No. 4,668,865. As shown in FIGS. 6A and 6B, this is formed into a cantilever 65 by epitaxially growing Si 63 after an electrode layer 62 is provided on a Si wafer 61 by doping an impurity, again doping an impurity into it to form an electrode 64, then removing a non-doped Si layer in between, forming a lead electrode 67 by vapor deposition. A piezoelectric thin film is used in structures such as an STM probe (IEEE Micro Electro Mechanical Systems, P188-199, February 1990) proposed by Quate et. al. of Stanford University. As shown in FIG. 7, this is formed by removing a part of the rear surface of the Si wafer to form a silicon membrane, stacking Al 72 and ZnO 73 sequentially on the surface of a bimorph cantilever, then removing the silicon membrane and an etching protecting layer (a silicon nitride film) on the surface of the wafer by reactive dry etching, thus forming a bimorph cantilever for STM tip displacement. A microtip for detecting the tunnel current is mounted on the free end portion of the top surface of the cantilever so that a satisfactory STM image can be obtained.
The observations of semiconductors, polymers or the like at atomic and molecular levels have been evaluated by use of a STM technique, and micromachining (E. E. Ehrichs, 4th International Conference on Scanning Tunneling Microscope/Spectroscopy, '89, S13-3) has been researched. Applications of recording and reproducing apparatuses in various fields has also been researched. In particular, there is an increasing demand for large capacity recording apparatuses for storing information upon which calculation is performed by a computer. Since microprocessors have become compact and its calculation performance capability increased with advances in the semiconductor process technology, there is a demand for recording apparatuses to be more compact. In order to satisfy these needs, a recording apparatus has been proposed which records and writes by varying the shape of a recording medium by applying a voltage from a converter formed of a microprobe. Such a microtip is provided on a driving means capable of finely adjusting the distance to a recording medium, for generating a tunnel current, the minimum recording area thereof being a 10 nm square.
A microtip of a STM is formed on a free end side of a cantilever, the cantilever being formed from several sub-cantilevers so as to flex independently of each other, further made integral with a semiconductor device. A recording and reproducing apparatus has been proposed having a cantilever with a microtip for detecting a tunnel current, an amplifier for amplifying the tunnel current, a multiplexer for selecting cantilever driving and tunnel current and the like mounted on one board.
A conventional cantilever itself is formed of multi-layers for the purpose of driving the scanning. The cantilever construction shown, for example, in FIG. 7, is a five-layer structure of piezoelectric members and electrodes. If different types of thin films are stacked or bonded together, internal stress inevitably occurs in the thin films. This is thought to occur in the interface because of differences in the thermal expansion coefficients and lattice constants thereof. Particularly, in thin films (thickness&lt;2 .mu.m), internal stress which occurs in the interface is a major problem. For this reason, it is impossible to strictly control stress values, and the cantilever formed by laminating thin films sometimes warp because of internal stress, making it impossible to control the driving of the cantilever with a high degree of accuracy. Moreover, since the cantilever is formed of piezoelectric substances, Young's modulus thereof is low, and therefore the natural frequency and rigidity thereof are not high. A low natural frequency and rigidity causes the scanning speed to be slow, and quite a few problems resulting from mechanical impact may occur.
Also, since the cantilever itself in the electrostatic driving type of U.S. Pat. No. 4,668,865 shown in FIG. 6 is used as an electrode, a two-layer structure of a cantilever and an insulating layer is necessary in order to insulate the microtip. The above case presents a problem in that parasitic capacitance is induced in the microtip because a microtip 68 and an electrode 64 are arranged with an insulating layer 66 in between. Moreover, in this method, the cantilever can be driven only perpendicular (Z-axis) to a Si wafer board 61.